1. Field of the Invention
The present invention relates to a universal motor. More specifically, the present invention discloses a universal motor that has an internal load for preventing excessive rotation of the motor when the motor has a light external load.
2. Description of the Prior Art
Universal motors are electric motors that, because of their ability to accept both AC and DC power sources, can be found quite readily in the market place, such as in cordless powered screwdrivers, blenders, etc. Universal motors are also in wide use in industry, as they have a simple, reliable design.
Please refer to FIG. 1. FIG. 1 is an exploded view diagram of a prior art universal motor 10. The universal motor 10 comprises a stator 20, a rotor 30 and end brackets 50 and 70. The rotor 30 has a shaft 32 onto which are mounted a commutator 34 and a rotor core 36. The rotor core 36 is wound with copper wire to form many individual groups of windings 38. Each of these windings 38 is electrically connected to a corresponding surface 35 on the commutator 34. The ends of the shaft 32 are rotatably fixed in bushings 52 and 72 of the end brackets 50 and 70, respectively. The end brackets 50 and 70 are firmly mounted onto the stator 20. Hence, the rotor core 36 rotates within the stator 20, and the commutator 34 rotates with the rotor core 36. The stator 20 also has many windings 22, which have leads 24 for delivering electrical power to the windings 22. The end bracket 50 comprises brush holders 60 for elastically holding brushes 62 in contact with the commutator 34. Electrical power is delivered by the brushes 62 to the commutator 34, and hence to the appropriate windings on the rotor core 36. Each brush holder 60 comprises a socket 61 into which a brush 62 is slidably disposed, a spring 64 for elastically pushing the brush 62 towards the commutator 34, and an external contact 66. The external contact 66 is electrically connected to the brush 62, and is used to deliver electrical power to the brush 62. Wires 68 lead off from the external contacts 66 to a power source (not shown).
When operating under a proper external load, the universal motor 10 will spin at a given RPM (revolutions per minute) with a given torque. Please refer to FIG. 2, which is a graph of torque versus RPM for the universal motor 10. Generally speaking, the relationship of torque versus RPM for the universal motor 10 is a linear one. Hence, when the universal motor 10 has a light external load, its rotational speed will increase dramatically. This rapid rotation of the universal motor 10 is a significant source of unwanted noise.
It is therefore a primary objective of this invention to provide a universal motor that has an internal load to prevent excessive rotation when the motor has a light external load.
The present invention, briefly summarized, discloses an internally loaded universal motor that has a stator, a rotor, brushes and a loading device for generating an internal load on the universal motor. The shaft has a commutator, and a rotor core adjacent to the commutator. The rotor core is electrically connected to the commutator and rotates within the stator. The brushes deliver electrical power to the commutator. The loading device prevents excessive rotation of the rotor when the universal motor has a low external load.
It is an advantage of the present invention that the internal load insures that the universal motor always has at least a slight load when there is a very low, or no, external load. By always providing a load on the universal motor, the rotational speed of the motor is kept down, which helps to reduce the noise emanating from the motor.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings.